Magnetic recording media are generally manufactured by forming magnetic layers on non-magnetic substrates. The magnetic layers contain magnetic powder in binders, to which various additives such as antistatic agents, lubricants, abrasives, dispersants, and stabilizers are added to provide a good profile of electromagnetic properties, durability, and reliability.
In recent years, the requirement of high density recording is imposed on magnetic recording media in accordance with the size reduction of the equipment.
One approach for increasing the recording density of magnetic recording media is to furnish magnetic powder having a higher coercive force, higher saturation magnetic flux density or smaller particle size. Also attempts have been made for achieving a higher packing density and a higher degree of orientation of magnetic powder as well as smoothing the surface and reducing the layer thickness. These attempts for higher recording density, however, have arisen many problems including losses of reliability and durability, causing magnetic head clogging, frequent occurrence of dropouts and deterioration of still performance.
It is known that reliability and durability can be improved through a proper choice of the type and amount of abrasive. Japanese Patent Application Kokai No. 57036/1986, for example, proposes to control the population or density of abrasive to at least 0.25 particles/.mu.m.sup.2 on the magnetic layer surface. In the case of magnetic layers which are reduced to a thickness of about 4 .mu.m or less for high density recording purposes, an increase of the density of abrasive by an ordinary distribution technique can lead to a loss of electromagnetic properties such as sensitivity and C/N and an increased abrasion of the associated magnetic head despite improved reliability and durability.
In turn, Japanese Patent Publication No. 14482/1979 proposes a magnetic layer of double layer structure for preventing magnetic head clogging and improving electro-magnetic properties. However, the magnetic layer contemplated therein is relatively thick as understood from the example in which two magnetic layers of 6 .mu.m and 4 .mu.m are stacked to form a composite magnetic layer of 10 .mu.m thick. That is, thin magnetic layers, say about 4 .mu.m or less, required for high density and long term recording are not borne in mind. This is partially because such extremely thin layers can be stacked with difficulty or at the sacrifice of productivity. Even if the upper layer to be stacked can be as thin as about 0.6 .mu.m, the abrasive is distributed uniformly in a thickness direction of the upper layer and at a relatively high density. In addition, such a thin upper layer cannot be effectively worked as by calendering. Consequently, electromagnetic properties are adversely affected and productivity is lost due to the complex manufacturing process.
As discussed above, high density recording media must meet ambivalent requirements of electromagnetic properties and reliability and durability. The prior art techniques for abrasive addition are difficult to find a compromise therebetween.